Process for surface treatment of iron and its alloys

ABSTRACT

TREATING THE SURFACE OF IRON AND ITS ALLOYS WITH AN AQUEOUS SOLUTION CONTAINING BROMIDE IONS OR IODIDE IONS AND PHOSPHORIC ACID RESULTS IN IMPROVED SURFACE FOR ADHESIVE JOINING AND FOR COATING WITH ORGANIC FINISHES. EFFECTIVENESS OF THE TREATMENT ON HIGHLY CORROSION RESISTANT STEELS IS SOMEWHAT IMPROVED BY THE ADDITION OF MINOR AMOUNTS OF SULFURIC ACID TO THE SOLUTION.

Jufly 27, 1971 H. N. mmwmn PROCESS FOR SURFACE TREATMENT OF IRON AND ITSALLOYS Filed Nov. 12,, 1968 ADHERENT ORGANIC LAYER IRON- CONTAININGSURFACE INVENTOR BY HIV l AZ/RA/V/ 3,595,708 PROCESS FOR SURFACETREATMENT OF IRON AND ITS ALLOYS Hargovind N. Vazirani, PassaicTownship, Morris County, N..I., assignors to Bell TelephoneLaboratories, Incorporated, Murray Hill, NJ.

lFiled Nov. 12, 1968, Ser. No. 774,930 Int. Cl. (123i 7/10 US. Cl.143-6.15 8 Claims ABSTRACT OF THE DISCLOSURE Treating the surface ofiron and its alloys with an aqueous solution containing bromide ions oriodide ions and phosphoric acid results in improved surface for adhesivejoining and for coating with organic finishes. Effectiveness of thetreatment on highly corrosion resistant steels is somewhat improved bythe addition of minor amounts of sulfuric acid to the solution.

BACKGROUND OF THE INVENTION (1) Field of the invention This inventionrelates to the surface treatment of iron and its alloys by the use of anacid aqueous solution containing bromide ions or iodide ions, in orderto improve joints subsequently formed with organic materials, and alsorelates to the joined product.

(2) Description of the prior art Joining iron and its alloys to organicmaterials is desirable in many cases, as for example, in the adhesivejoining and protective coating of steel structural parts. Corrosionproducts ordinarily present on the surface of the metal interfere withsuch joining and have in the past been removed both chemically, as forexample, by hydrochloric acid treatment, and mechanically, as forexample, by abrasive blasting.

Mechanical abrasion is generally undesirable because it is wasteful ofthe metal surface and produces a rough texture which gives rise tolocalized stresses in subsequently formed joints when placed understress.

Chemical cleaning is less damaging to the surface of the metal. Inaddition, the corrosion products may be replaced by a chemicallyproduced oxide layer which prevents further corrosion and provides asurface suitable for joining. This layer has been achieved, for example,by the use of an oxidizing agent such as hydrogen peroxide, as disclosedin US. Pat. 2,923,608. However, joints subsequently formed on suchsurfaces often degrade with time resulting in eventual joint failures.Depending on the temperature, stress and humidity levels, such failuresmay occur after only a few days.

SUMMARY OF THE INVENTION This invention is essentially a joiningtechnique in which the surfaces of iron and its alloys are treated witha solution containing bromide ions or iodide ions and phosphoric acidprior to joining. Such treatment results in removal of corrosionproducts and their replacement by a mechanically strong protective layerthus enabling joints to be formed which exhibit excellent initialstrengths and which resist degradation with time.

The invention encompasses treating solutions containing minor additionof sulfuric acid, preferred for treatment of high corrosion-resistantsteels.

Since the surface to be treated ordinarily contains contaminants such asdirt, grease and corrosion products, it is usually necessary to removethese prior to the surface treatment and to aid the practitioners,exemplary procedures are briefly described.

United States 3,595,708 Patented July 27, 1971 BRIEF DESCRIPTION OF THEDRAWING DETAILED DESCRIPTION This process applied to iron and any of italloys containing at least 50% iron, and has as its principal object theremoval of dirt and corrosion products from the metal surface andreplacement with a chemically produced layer. Since an object of thesurface treatment is to remove oxide scale, any advantage to be gainedby preliminary cleaning of the metal surface will ordinarily reside inthe reduced rate of contamination of the treating solution, replacementof which may be both costly and time consuming.

Preliminary cleaning is by methods well known in the art. Thus, thefollowing description of preliminary cleaning is intended to beexemplary and not limiting.

Preliminary cleaning is generally divided into degreasing and descaling.For example, degreasing is generally effective in removing only oils andgrease and is ineffective in removing corrosion products such asnaturally formed oxide scale. Descaling, which may be either mechanicalor chemical, will, however, generally remove substantially all of thesurface contamination. However, descaling chemically without first.degreasing may result in rapid contamination of the solution.

Removal of oils and grease may be accomplished by the use of organicsolvents such as alcohols, ketones and chlorinated solvents such astrichloroethylene and perchlorethylene, or by the use of well knownalkaline cleaners.

Descaling is usually accomplished by the use of acid solutions or bymechanical abrasion. However, where joints of high initial strength andhigh resistance to degradation are desired, it is preferred to avoidmechanical abrasion, strong chemical etchants, or other descalingmethods which would result in rough or pitted metal surfaces.

A more complete description of degreasing and descaling methods may befound in Protective Coatings for Metals, third edition, AmericanChemical Society, Monograph 163, by R. M. Burns and W. W. Bradley, pp.27 to 54, in Reinhold (1967).

Once the preliminary removal of surface contamination has been effected,the clean metal surface should be either treated promptly to form thedesired protective layer or stored under noncorrosive conditions untiltreatment in order to realize any advantage gained, since exposure ofthe clean surface to a nonprotective atmosphere will soon result inreformation of corrosion products.

Formation of the protective layer may be carried out by contacting themetal surface with a treating solution containing bromide or iodide ionsin the range from 0.001% by weight to saturation although from 1 to 10%of iodide is preferred for optimum results. The ions may be introducedin combination with the Group I alkali metals; lithium, sodium,potassium, rubidium and cesium or the Group II alkaline earth metals;beryllium, magnesium, calcium, strontium and barium.

The phosphoric acid should generally be present in the treating solutionin a concentration sufficient to etch the metal uniformly typically 30to below which uneven etching or even pitting is likely to occur, andabove which the rate of attack of the metal surface is impracticallyslow. A concentration of from 40 to 60% of H PO is preferred, based onthese considerations.

The phosphoric acid solution containing iodide ions is intendedprimarily for use with the less corrosion-resistant steels, such as coldrolled steels and the high 3 strength steels. Highly corrosion-resistantor stainless steels, not only resist the action of the treatingsolution, but also generally require less critical surface treatmentpreparatory to joining. However, if it is desired to treat high- 1ycorrosion-resistant surfaces, a strongly etching acid, such as H SO maybe added to the treating solution in amounts up to about beyond whichformation of a rough, pitted surface is likely.

The degree of attack of the surface will in general increase withincreasing time of treatment or temperature of solution. Accordingly, athigh temperatures, relatively short treatment times should be used. At230 F., constituting an upper limit beyond which boiling ordecomposition of the solution will occur, the time of treatment shouldgenerally not exceed two minutes, while at room temperature, treatmenttimes of about ten minutes are tolerable. For optimum results, apreferred range is from three to five minutes at about 160 to 200 F.

The following examples compare adhesive joint strengths obtained by theinventive treatment and by various other treatments.

EXAMPLE 1 Four sets of standard lap joints were made from speciments ofNo. 1010 (about 0.1% carbon) cold rolled steel (having dimensions ofabout one inch in width and sixty-three mils in thickness) and an epoxyadhesive, claimed to give strong joints without surface treating.Treatment prior to joining was as follows: Set 1 was not treated. All ofthe specimens in sets 2, 3, and 4 were vapor degreased. Set 3 was thengrit blasted with alumina powder. Set 4 was treated for five minutes ina solution containing 5% by weight of KI and 50% by volume conc. H PO at180 F., and rinsed with deionized water. The results are shown in Table1 as the initial joint strength in pounds per square inch and time tofailure when a static load of 1500 pounds per square inch was applied inan ambient atmosphere of 95 F. at 90% relative humidity.

TABLE 1 Inltial joint strength (p.s.i.) 2, 900 2, 910 4, 260 4, 470 Daysto failure, 1,500 p.s.i., static stress. 2 19 101 170 EXAMPLE 2 TABLE 2Initial joint strength (p.s.i.) Days to failure, 4,000 p.s.l

The results of Tables 1 and 2 indicate that strong joints, desirableunder static loading, are obtainable by use of the inventive treatmenton cold rolled steel. The following example shows the beneficial effectsof addition of H 80 to the treating solution and of elevated treatmenttemperatures in the case of highly corrosionresistant steel.

EXAMPLE 3 Three sets of standard lap joints were made from specimens ofNo. 302 steel (approximate composition; 0.15% carbon, 2% manganese, 1%silicon 17 to 19% chromium, 8 to nickel, remainder iron) as inExample 1. Treatment prior to joining was as follows: All specimens ineach set were vapor degreased and alkaline cleaned.

TABLE 3 Initial joint strength (p.s.i 6, 400 6, 730 7, 450 Days tofailure, 4,000 p.s.i 27 60 104 The invention has been described in termsof improving joints between iron and its alloys and organic materials.Such organic materials include adhesives, coatings and pottingcompounds. Examples of adhesive materials are epoxies and modified epoxyresins, nitrile rubber phenolics, polyvinyl butyrals and polyvinylformals. Examples of coating materials are alkyds and modified alkyds,acrylics, polyesters and urethanes. Potting compounds includepolyurethanes, silicones and epoxies.

The following example indicates that coatings adhere well to metalsurfaces treated according to the invention.

EXAMPLE 4 Three sets of No. 1010 cold rolled steel panels were treatedas follows: All sets were vapor degreased. Set 2 was treated with a zincphosphate solution and rinsed with a chromic acid solution, a commonlyused treatment prior to coating. Set 3 was treated as was Set 4 inExample 1. All sets were then coated with about a /2 mil thick primerfollowed by about a 5 mil thick layer of a vinyl acrylic organosol. Thecoating was then fused at about 360 F. for twenty minutes, a standardcoating technique. Results are shown in Table 4 in terms of visualappearance of coatings after salt spray according to ASTM test B117 andhumidity cycling.

Referring now to the drawing, there is shown a joined product preparedin accordance with the inventive process. Shown in the figure is an ironor iron alloy body 1, joined to an organic body 2.

In FIG. 2 is shown an iron or iron alloy body 10, joined to an organicbody 11, and further joined to a metal body 12.

What is claimed is:

1. A method for preparing a surface containing at least 50% by weightiron for joining to an organic material, characterized in that saidmethod comprises the step of contacting said surface with an aqueoussolution consisting essentially of phosphoric acid in the amount of from40 to 60% by weight and at least one member from the group consisting ofbromide ions and iodide ions, said member being present in the amount offrom 1 to 10% by weight and said member being introduced into thesolution as a compound of an element selected from Groups I and II ofthe Periodic Table.

2. The method of claim 1 in which said solution additionally contains upto 5% by weight of sulfuric acid.

3. The method of claim 1 in which the step of contacting said surfacewith said solution is carried out at a temperature of up to 230 F. forup to two minutes 4. The method of claim 3 in which said step ofcontacting said surface with said solution is carried out at atemperature of from 160 to 200 F. for from three to five minutes.

5. The method of claim 1 in which the step of contacting said surfacewith said solution is preceded by descaling.

6. The method of claim 1 in which the step of contacting said surfacewith said solution is followed by adhesive joining of an organicmaterial to said surface.

7. The method of claim 6 in which the joining is for 8. Product producedby the method of claim 1.

References Cited UNITED STATES PATENTS 1/1968 Forsberg 1486.l5 7/1950Amundsen 1486.15

10 RALPH s. KENDALL, Primary Examiner US. Cl. X.R. 1486.17

